The present invention relates to a hydraulic modulator valve, and more particularly to a novel improvement for the type of hydraulic modulator valve, of which the stepped spool makes axial displacements to regulate fluid pressure.
To modulate line pressure within a control system for a hydraulically operated mechanism, such as an automatic hydraulic frictional engaging mechanism of a forklift truck or other vehicles, there has been introduced a modulator valve including a stepped spool axially slidable therein. The stepped spool is provided with a large diameter portion to be hermetically engaged in the bore of the valve and a small diameter portion cooperating with inlet and outlet ports drilled on the valve casing so as to form an annular throttle passage.
The axial displacements of the spool control the line pressure to be supplied to a hydraulic actuator for the hydraulically operated mechanism. With the type of modulator valve as explained, the pressure control operation may be realized essentially in a desired condition in its initial and final stages. The problem is that no ideal fluid pressure control characteristics are obtainable in the middle stage which is important in the control operation particularly for the actuators of the hydraulically operated mechanism for vehicles. That is, the factors for pressure control are the difference in diameter between the large and small diameter portions of the spool and the axial length of the small diameter portion. Only with these factors, there exists a distinct limitation in controlling the modulating characteristics by this type of modulator valve.
Furthermore, in actual usage of the conventional type of modulator valve, the displacement of the spool is manually controlled for fluid pressure modulating operation. This causes unsmooth operation of the hydraulic actuators because of time gaps produced by differential operations between the valve and the actuator.